Electroacoustic transducers are generally comprised of an array of active elements in the form of piezoelectric crystals that are mounted in parallel, spaced relationship on the surface of a base of sound-absorbing material. The base is typically constructed of a backing material that exhibits particular acoustical characteristics. For example, the backing material is typically formed by molding a composition of a material having a high acoustical impedance, such as tungsten powder, and an acoustically-absorbing binder so as to substantially eliminate spurious acoustic reflections.
In constructing such a transducer, it is customary to adhere the back of a large crystal to the surface of the base and saw through it in parallel spaced planes so as to form the separate crystals of the array. Acoustic transducer arrays, and in particular ultrasonic transducer arrays, may be arranged in a number of configurations including linear, one-dimensional arrays, matrix two dimensional arrays, annular ring arrays, etc. Harmful coupling between the elements of the array by surface waves is substantially reduced by extending the cuts into the base. The backing material therefore must be sufficiently rigid so as to maintain the crystals in proper position.
It is, therefore, desirable that the backing material offer certain mechanical and acoustical characteristics: rigidity, for structural support of the elements in an array; selectable acoustic impedance, for controlling or eliminating the reflections at back surfaces of the elements, to achieve a desired balance between output power and image sharpness; and acoustical attenuation, such that acoustic signals exiting the back of the active elements be substantially attenuated so that image-degrading reflections of such signals are not returned to the transducer element.
The advent of ever-smaller ultrasonic transducers has imposed a need for highly attenuative backing materials because the thickness of the base must be reduced. However, it has proven difficult to achieve a backing material that, in addition to providing adequate structural support, can be constructed as a thin member that is highly attenuative.
The conventional approach is to provide a backing material in the form of a rigid resinous matrix into which are dispersed attenuative particles. A backing material might, for example, be formed of an epoxy material having acoustic absorbers and scatterers such as tungsten, silica, chloroprene particles, or air bubbles. Known additive particles have been formulated from sintered metal powders, siliceous powders, and other materials that exhibit a high acoustic velocity and increase the rigidity of the matrix.
As disclosed in U.S. Pat. No. 4,382,201, tungsten and polyvinyl chloride composites have been prepared containing relatively large tungsten particles (50 micron diameter) which act as scattering centers, thereby increasing the attenuation in the matrix. The acoustic waves are said to be reflected by the large particles and have a longer path length. This system can be ineffective at attenuating frequencies greater than about 4.5 MHz. At the higher frequencies the large particles reflect increasing amounts of acoustic energy back into the transducer active element, and as a result the noise level increases.
Another approach is disclosed in U.S. Pat. No. 5,297,553 wherein the backing material includes a plurality of rigid metal, ceramic, polymeric, or polymer-coated particles that are said to be fused into a macroscopically rigid structure, which is then impregnated with an attenuative filler.
The foregoing approaches can be difficult, expensive, or otherwise impractical for some applications. For example, attenuative (i.e., soft) particles are difficult to prepare in very fine sizes. Certain soft particles are not easily dispersed to a uniform distribution within a resinous filler, and often do not maintain proper dispersion while the filler hardens. Hardened scattering particles, such as tungsten particles, sized at one-tenth of a wavelength or greater, have been uniformly distributed throughout a backing material in order to improve its ability to scatter acoustic energy, but as noted in the prior art, the large particles damage a saw blade used to partition the crystal and a portion of the base into an array of individual elements.
In order to avoid acoustical reflection at the interface of the base and the array and to avoid using a thick layer of adhesive in attaching the back of the array to the base, it is desirable that the interface of the array and base is smooth and uniform. The base may be prepared by polishing, but it has been found that tungsten and similar particles can be pulled entirely out of the binder, thus resulting in a rough surface filled with small craters which cause undesired reflections of acoustic energy.
A need thus exists for an inexpensive, easily-formed, and practical backing material that can provide the desired acoustical properties, yet also provide the desired mechanical properties, such that a base formed from the backing material can act as a rigid support for the active elements.